Monocone and bicone (also termed biconical herein) antennas are well-known in the art. Many variations on the basic design of the monocone (cone, feed and ground plane) and bicone (pair of cones, feed and balun, with or without ground plane) are known. Applicant has developed an innovative “coneless” design that provides comparable or better performance relative to the known monocone and bicone antennas. The coneless design preserves the desirable performance of a conical antenna, but achieves advancement in antenna capability that has been desired, but not realized, for many years. The present invention is a simple, robust and inexpensive multifunctional antenna system that provides high gain over a large bandwidth. The innovative shape of the feed region of the present invention, having “tapered feed points” disposed substantially at the circumference of the cylindrical antenna structure, opens up the typical conic tip region of known monocone and bicone designs. The one or more tapered feed points replace the single feed/single conic tip that typically feeds known monocone antennas or the single feed/two conic tips of known bicone antennas. For optimal performance, the circumferential spacing of the tapered feed points is less than half a wavelength at the highest frequency of operation.
In order to improve bandwidth coverage, as well as gain, it is well-known to combine multiple antennas. Applicant has previously disclosed an ultra-broadband antenna system (U.S. Pat. No. 7,339,542, assigned to Assignee of the present invention) that combines an asymmetrical dipole (covering intermediate frequencies), fed with a biconical dipole (covering high frequencies), that together act as a monopole (covering low frequencies), all in a single tubular structure. The design of U.S. Pat. No. 7,339,542, including the use of a choke to limit interference, resulted in an ultra-broadband antenna system with a frequency span greater than 500:1. Nonetheless, this antenna system was limited by the very small opening in the conic tips of the biconical dipole, which resulted in coupling and interference. In order to combine additional elements with this ultra-broadband antenna system, Applicant has applied the coneless shape of the herein-described monocone to the biconical antenna element. The cut-away or shaped design of the feed region of the present invention opens up the typical “cone” of the prior art conical antennas, making a larger opening in the center of the antenna structure. Indeed, the diameter of the coneless element is substantially as large as that of the cylinder of the tubular antenna structure. This allows antenna feed lines or a wide variety of cables, such as coaxial, power, digital, fiber optic, wire, etc., as well as piping, tubing, actuators or other structures, to be run through the center of the antenna with minimal to no interference with the standalone antenna performance. For the biconical antenna of the present invention, the coneless elements may be aligned, or the elements may be clocked to improve performance in azimuth.
Another approach to providing wider bandwidth and improving gain has been to stack biconical radiators. Those skilled in the art have long studied the cone angle, overall length of the antenna, and diameter of the biconical elements in attempts to provide impedance matching of the antenna elements. An unsolved problem has been providing the feed to the stacked biconical structures without interfering with the RF performance of the lower biconical element. The innovative design of the present invention provides the same impedance matching and RF performance of known single feed point biconical structures, by positioning the one or more tapered feed points on the circumference of the cylindrical feed region. Stacking two coneless biconical elements results in higher gain at a given bandwidth; the present invention allows stacking of three, four or even more coneless biconical elements, for even higher gain, which provides the advantages of both increased range and reduced power requirements. To provide a wider frequency range, elements of differing diameters and/or differing length may also be stacked, without degradation in performance of the individual elements. At the same time that it provides greater bandwidth and/or higher gain, the innovation of present invention can allow reduction in the size of the antenna system, such as height, footprint, or diameter, or allow the system to be made conformal.
Thus, the innovative design of the coneless elements not only provides the physical space for feed lines to be run through the center of the tubular antenna structure, it also allows a wide range of devices to transmit and receive RF, audio, video and other optical frequencies, or other signals without interfering with the performance of the antenna system. In addition, non-electrical feeds, such as hydraulic, pneumatic and mechanical controls or actuators, and gas, liquid or solid material transfer systems, may also be run through the center of the antenna without degrading performance. The innovation of the present invention thus has many practical applications. Devices such as cameras, IR sensors, GPS devices, lights, audio equipment, radar equipment and communications equipment all may be mounted on the top of a multiple element, tubular antenna system that has a relatively small footprint. Where preferable, such devices may also be mounted in between multiple antenna elements. In many situations, this may obviate the need for multiple (separate) antennas, which otherwise would have to be placed apart in order not to interfere with each other.
By allowing the collinear and coaxial stacking of multiple antennas, the present invention is able to provide an antenna system with virtually unlimited bandwidth. Further, the present invention allows for both directional and omni-directional coverage, depending on the type of antennas combined.
Applications for the present invention, allowing for a wide variety of multiple stacked antennas and/or other devices, include placement on land vehicles, ships, planes, helicopters or spacecraft; land-based or sea-based locations; as well as man-portable uses.
The known art of antennas is voluminous. Applicant believes that the present invention may distinguished from the relevant prior art as follows. Typical known conical and biconical antennas, exemplified by the work of Carter, such as U.S. Pat. No. 2,175,252, disclose a single conical feed point that excites the cone-shaped radiator, which may be a single cone disposed above ground, or two cones about the same axis forming a bicone. The conical shape provides an impedance appearing almost as a pure resistance, or has no reactive component with variation in frequency, thus is useful over a wide frequency range. U.S. Pat. No. 2,416,698 to King discloses a single biconical with one feed point, having a hollow central cylinder. U.S. Pat. No. 2,543,130 to Robertson discloses yet another early biconical antenna, having a hollow pipe guide connected to a horn-shaped radiator for improved impedance matching. Like the present invention, monocones and bicones give broadband performance. Unlike the present invention, however, the foregoing designs do not permit the stacking of multiple antenna elements or other devices, because feed lines or cables cannot be run from the hollow central elements through the feed region without causing interference.
Another type of known antenna which does permit stacked collinear elements employs a traveling wave feed system. U.S. Pat. No. 2,471,021 to Bradley discloses a plurality of stacked biconical horn antennas, which use a driving network to couple into a circular wave guide through symmetrically arranged slots. U.S. Pat. No. 3,605,099 to Griffith discloses an antenna with stacked pairs of frustoconical reflector elements attached to a central hollow support member containing a central conductor. Feed is via traveling wave transmission through slots, connecting adjustable probes between the slots and the central conductor. U.S. Pat. No. 4,225,869 to Lohrmann discloses a multicone antenna having ¼ wavelength cones at each slot of a slotted ring antenna. U.S. Pat. No. 6,593,892 to Honda et al. discloses stacked biconical elements with a single center feed line. This class of antennas can be relatively broadband, and permit stacking of collinear biconical elements. The feed method of such systems is fundamentally different from that of the present invention, however, as the traveling wave is not an independent direct feed to each element. Further, all antennas using traveling wave feed are roughly the same type and size, whereas the present invention may combine a wide range of different antennas and different devices. Although traveling wave antenna systems potentially could accommodate additional devices in the collinear array by running cables or piping through the central conductor, energy is bled off as it proceeds through the slotted structure and therefore the feed to each element is not isolated, as is the case in the present invention. The functionality is limited because it does not have full control over phase and amplitude weighting. This approach also does not allow the ability to use antennas that perform at different frequency bands or perform independently of each other.
An alternate approach that allows stacking of antenna elements is to choke the antenna feed or route the feed externally. U.S. Pat. No. 3,727,231 to Galloway et al. discloses a collinear dipole array antenna with independent feeds using a narrowband technique which connects a coaxial cable to an external transmission line, in combination with λ/4 chokes for isolation, allowing a maximum of two elements.
U.S. Pat. No. 4,410,893 to Griffee discloses a collinear dual dipole antenna, also using a narrowband technique to jump the gap between two biconicals. U.S. Pat. No. 5,534,880 to Button et al. discloses multiple stacked bicone antennas with a bundle of transmission lines helically wound about the cylindrical periphery of the biconical antennas. This design uses exterior routing of cable to minimize the interference problems of passing the cables up the central column. U.S. Pat. No. 6,268,834 to Josypenko discloses multiple bicone antennas wherein the feed cable is led to a center point, then directed radially along the cone to an inductive short, through the inductive short, then directed along the surface of another cone to the center line. Again, this exterior routing of the cables minimizes the pattern perturbation. As exemplified by the foregoing, such designs do allow stacked elements and do have direct feeds to the antenna elements, but unlike the present invention, employ either a choked, centrally-fed system that permits only a relatively narrowband performance, or an externally-routed feed system for broader band operation.
U.S. Pat. No. 7,170,463 to Seavey discloses a broadband communications antenna system with center-fed, stacked dipole elements having conical shaped feed points and isolated with ferrite chokes (coiled inductors across the junction). The chokes are in close proximity to the actual feed, thus reducing the radiation efficiency of the antenna system. U.S. Patent Application Publication No. 2008/0143629 to Apostolos discloses a coaxial multi-band antenna combining a VHF, a UHF and a satellite antenna on a common radiating element, using meander line or ferrite chokes to isolate the feeds for each antenna. Unlike the narrowband choked designs of Galloway and Griffee, Seavey's and Apostolos' systems are relatively broadband, like that of Applicant's U.S. Pat. No. 7,339,542. The design of the present invention, however, obviates the need for chokes to isolate the feeds for stacked elements, thus is an improvement over all choked configurations and provides significantly greater efficiency and bandwidth.
In yet another approach, stacked, collinear and relatively broadband antenna systems are made possible by using waveguide structures to provide independent separate feeds to the antenna elements. U.S. Pat. No. 4,477,812 to Frisbee, Jr. et al. discloses a collinear array receiver system with a dipole antenna mounted atop the array. Using slot excitation, however, a system such as Frisbee, Jr.'s must be electrically large, on the order of tens of wavelengths, in order to allow space for transmission via slot. The present invention, in comparison, is on the order of one wavelength, and therefore provides the desired performance using a greatly reduced footprint. U.S. Pat. No. 6,864,853 to Judd et al. discloses stacked elements (a dipole combined with patch antenna elements) in a unitary structure that provides both directional and omnidirectional beam coverage, as well as a stack of bi-conical elements each having a frusto-conical reflector portion that together form a central passageway containing a feed system of coaxial cables. The omnidirectional array of bi-conical antennas configured end-to-end appears to use a waveguide feed structure, that, again, would be electrically large. Like the foregoing, the present invention utilizes independent separate feeds for each antenna element, but does not require the electrically large conical radiators of these waveguide-fed structures.
Finally, the prior art includes another antenna type that allows stacking of coaxial and collinear antennas. Termed “CoCo” antennas, these systems incorporate the feed system as part of the radiating structure. Examples are found in U.S. Pat. No. 6,947,006 to Diximus et al., which discloses a stacked collinear narrowband antenna that radiates on the transmission line structure, and in the 2006 paper “Generalized CoCo Antennas” by B. Notaro{hacek over (s)}, M. Djordjević and Z. Popović, which presents recent contributions to the theory and design of transmission-line antennas. This paper notes that the “CoCo antenna is inherently narrowband, and as such intended for practically single-frequency operation,” and therefore has a very different functionality from the present invention. As well, the feed mechanism of CoCo antennas is distinct from that of the present invention, which as described above, has the transmission line structure isolated from the radiating structure.
Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.